The present invention relates to a method and apparatus for measuring ellipsometric parameters, which ellipsometric parameters are used to measure the thickness of a thin film accurately, and, more particularly, to a measuring method for ellipsometric parameters and an ellipsometer designed to select an optimal measurement condition automatically.
As a method of measuring the thickness of a thin film, ellipsometry is used. In this method, a change in polarization state upon reflection of a beam by a sample surface, i.e., a ratio .rho. between a reflectance Rp of a light component (P component) parallel to the incident plane of an electric field vector and a reflectance Rs of a light component (S component) perpendicular thereto, is measured according to equation (1), and a film thickness d is obtained in accordance with a predetermined relationship between the already obtained polarization reflectance ratio .rho. and the film thickness d: EQU .rho.=Rp/Rs=tan .psi. exp[j.DELTA.] (1)
In this case, since the polarization reflectance ratio .rho. is represented by a complex number as indicated by equation (1), two ellipsometric parameters, i.e., an amplitude ratio .psi. and a phase difference .DELTA., must be obtained.
As a conventional method of obtaining these ellipsometric parameters .psi. and .DELTA., a rotating analyzer method is known. In this method, for example, a polarized beam is radiated from a light source onto a measurement target at a predetermined angle, and a reflected beam from the measurement target, which is elliptically polarized, is guided to a light-receiving unit through a rotating analyzer. Subsequently, the ellipsometric parameters are calculated on the basis of the optical intensity signal waveforms obtained by the light-receiving unit at this time.
However, in order to execute one measuring operation, the analyzer must be rotated once and the resulting optical intensity signals must be observed. This rotation requires a predetermined period of time or more. Therefore, it is impossible to measure a film thickness on a measurement target which is moving at high speed. In addition, the presence of a mechanical movable portion increases the size of the apparatus itself. For this reason, the apparatus cannot be installed on a production line in a factory to perform on-line measurement of measurement targets, e.g., continuously supplied measurement targets.
In order to eliminate such inconveniences, a 3-channel ellipsometer, in which movable parts are eliminated, (Published Unexamined Japanese Patent Application Nos. 63-36105 and 1-28509) has been developed, as shown in FIG. 18.
For example, a beam having a single wavelength, output from a light source 1 constituted by, a laser source, is linearly polarized by a polarizer 2 and is incident on a sample surface 3 as a measurement target at a predetermined angle .phi.. Assume that, on the sample surface 3, an incident plane is parallel to the surface of the drawing, and that a direction parallel to the surface of the drawing is defined as a P direction; and a direction perpendicular to the surface of the drawing, an S direction. A reflected beam from the sample surface 3 is split into three beams by three non-polarizing beam splitters 4a, 4b, and 4c. Each of the beam splitters 4a to 4c is constituted by an optically isotropic, transparent member. In addition, the beam splitters 4a to 4c are fixed to be parallel to each other.
A first beam transmitted through the two beam splitters 4a and 4b is incident on a first light-receiving unit 7a through a first analyzer 5a and a condenser lens 6a. The first light-receiving unit 7a converts an optical intensity Ia of the beam into an electrical signal. Similarly, a second beam transmitted through the beam splitter 4a and reflected by the next beam splitter 4b is incident on a second light-receiving unit 7b through a second analyzer 5b and a condenser lens 6b. The second light-receiving unit 7b converts an optical intensity Ib of the beam into an electrical signal. In addition, a third beam reflected by the beam splitter 4a and transmitted through the next beam splitter 4c is incident on a third light-receiving unit 7c through a third analyzer 5c and a condenser lens 6c.
The third light-receiving unit 7c converts an optical intensity Ic of the beam into an electrical signal.
Each of the analyzers 5a to 5c serves as an element for checking the presence/absence of polarization and a polarization direction, and has the same constitution as that of the polarizer. Each of the analyzers 5a to 5c transmits only a light component which oscillates in a set direction. The polarization direction of the first analyzer 5a is set in a reference direction (an azimuth of 0.degree.). The polarization direction of the second analyzer 5b is set to be inclined at an angle of +45.degree. with respect to the reference direction. The polarization direction of the third analyzer 5c is set to be inclined at an angle of -45.degree. with respect to the reference direction. Note that the reference direction is a direction in which a direction (P direction) parallel to the incident plane of a beam incident on the sample surface 3 is defined as an azimuth of 0.degree., when viewed from the direction of the light-receiving unit 7a, as indicated by an arrow a in FIG. 18.
If, therefore, the light reflected by the sample surface 3 is elliptically polarized, as shown in FIG. 19, the first optical intensity Ia obtained by the first light-receiving unit 7a represents the amplitude, of the elliptically polarized beam shown in FIG. 19, which is orthographically projected on the axis of abscissa (0.degree. direction). The second optical intensity Ib obtained by the second light-receiving unit 7b represents the amplitude, of the elliptically polarized beam, which is orthographically projected on a line inclined at an angle of +45.degree.. The third optical intensity Ic obtained by the third light-receiving unit 7c represents the amplitude, of the elliptically polarized beam, which is orthographically projected on a line inclined at an angle of -45.degree..
The above-mentioned ellipsometric parameters .psi. and .DELTA. are the amplitude ratio .psi. and the phase difference .DELTA. between the P and S components of the reflected beam from the sample surface 3, which is elliptically polarized, as shown in FIG. 19. Simple geometrical examination reveals that the ellipsometric parameters can be obtained according to equations (2) and (3): EQU cos .DELTA.=(Ib-Ic)/(2Ia){Ia/(Ib+Ic-Ia)}.sup.1/2 ( 2) EQU tan .psi.=(.sigma.1.multidot..sigma.2){Ia/(Ib+Ic-Ia)}.sup.1/2( 3)
Note that the amplitude reflectance ratio .sigma.1 and the amplitude transmittance ratio .sigma.2 in the direction of each of the beam splitters 4a to 4c are inherent values, which are obtained in advance by radiating a test beam having a known elliptically polarized beam onto each of the beam splitters 4a to 4c.
When the ellipsometric parameters .psi. and .DELTA. are obtained in this manner, the film thickness d is obtained by using another equation.
However, in the conventional ellipsometer shown in FIG. 18, the following problems are still posed.
The optical intensities detected by the light-receiving units 7a to 7c greatly change depending on the shape of the elliptically polarized beam of the reflected beam from the sample surface 3, as shown in FIG. 19. For example, as the elliptic shape shown in FIG. 19 becomes flatter, only the third optical intensity Ic is greatly reduced as compared with the other optical intensities Ia and Ib.
In order to calculate the above-mentioned ellipsometric parameters .psi. and .DELTA. by, e.g., a computer, the optical intensities Ia to Ic must be converted into digital values by an A/D converter. If, therefore, only one optical intensity is small, the number of effective digits of the A/D-converted values is decreased, resulting in a large error. As a result, the precision of the calculated ellipsometric parameters .psi. and .DELTA. decreases, and hence the measurement precision of the finally obtained film thickness d decreases.
When optical intensity Ia takes a value close to "0", both the numerator and denominator of the fraction at the right side of Equation (2) also take values, close to "0", thus decreasing the calculation accuracy.
As described above, although the 3-channel ellipsometer shown in FIG. 18 is a very useful apparatus for measuring a film thickness on a measurement target at high speed because it has no movable portions, its measurement precision may become lower than that of the above-described ellipsometer using the rotating analyzer depending on the type of measurement target.
Note that if the number of measuring operations at the same measurement point is increased, and the average of the measurement results is obtained, a reduction in error can be achieved to some extent. However, repetitive measurement of the same measurement point prolongs the overall measurement time and cannot be applied to a measurement target which moves at high speed in the process of, e.g., film thickness test in a production line in a factory.
The present invention has been made in consideration of the above situation, and has as its object to provide a measuring method for ellipsometric parameters and an ellipsometer, in which a reflected beam having an elliptically polarized beam reflected by a measurement target is divided into four different polarized light components, the optical intensities of the respective polarized light components are detected, and a low optical intensity of the detected optical intensities is omitted, thereby calculating ellipsometric parameters by using only high optical intensities, and achieving a great increase in film thickness measurement precision while maintaining a high measurement speed.